Laminated sheets for printed circuit boards are obtained by stacking a prescribed number of prepregs comprising a resin composition with an electrical insulating property as the matrix, and heating and pressing the stack to form an integrated unit. Also, metal-clad laminated sheets are used when forming printed circuits by a subtractive process in the fabrication of printed circuit boards. Such metal-clad laminated sheets are manufactured by stacking metal foil such as copper foil on the prepreg surface (one or both sides), and heating and pressing the stack.
Thermosetting resins such as phenol resins, epoxy resins, polyimide resins, bismaleimide-triazine resins and the like are widely used as resins with electrical insulating properties. Thermoplastic resins such as fluorine resins or polyphenylene ether resins are also sometimes used.
However, the advancing development of data terminal devices such as personal computers and cellular phones has led to reduced sizes and higher densities of the printed circuit boards mounted therein. The mounting forms range from pin insertion types to surface mounting types, and are gradually shifting toward area arrays such as BGA (ball grid arrays) employing plastic substrates.
For a substrate on which a bare chip such as BGA is directly mounted, connection between the chip and substrate is usually accomplished by wire bonding which employs thermosonic bonding. Bare chip-mounted substrates are thus exposed to high temperatures of 150° C. and above, and the electrical insulating resins must therefore have a certain degree of heat resistance.
Such substrates are also required to have “repairability” so that the once mounted chips can be removed. This requires approximately the same amount of heat as for mounting of the chips, while the chip must be remounted later on the substrate and subjected to further heat treatment. Consequently, “repairable” substrates must exhibit thermal shock resistance against high temperature cycles. Conventional insulating resins have also sometimes exhibited peeling between the resins and fiber base materials.
For printed circuit boards there have been proposed prepregs comprising a fiber base material impregnated with a resin composition with polyamideimide as an essential component, in order to improve the intricate wiring formability in addition to thermal shock resistance, reflow resistance and crack resistance (for example, see Patent document 1). There have also been proposed heat resistant substrates comprising a fiber base material impregnated with a resin composition composed of a silicone-modified polyimide resin and a thermosetting resin (for example, see Patent document 2).
In addition, with the increasing miniaturization and high performance of electronic devices it has become necessary to house printed circuit boards with parts mounted in more limited spaces. Methods have therefore been adopted that involve disposing a plurality of printed circuit boards in a stack and connecting them together with a wire harness or flexible wiring board (for example, see Patent document 3). In some cases, rigid-flex substrates are used which are multilayer stacks comprising polyimide-based flexible substrates and conventional rigid boards (for example, see Patent document 4).    [Patent document 1] Japanese Unexamined Patent Publication No. 2003-55486.    [Patent document 2] Japanese Unexamined Patent Publication HEI No. 8-193139.    [Patent document 3] Japanese Unexamined Patent Publication No. 2002-064271.    [Patent document 4] Japanese Unexamined Patent Publication HEI No. 6-302962